linux-kernel - Re: [PATCH 2/2] PM / Domains: Avoid infinite loops in attach/detach code

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Date:	Wed, 24 Jun 2015 10:33:54 +0200
From:	Ulf Hansson <ulf.hansson@...aro.org>
To:	Geert Uytterhoeven <geert@...ux-m68k.org>
Cc:	Geert Uytterhoeven <geert+renesas@...der.be>,
	Daniel Lezcano <daniel.lezcano@...aro.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	"Rafael J. Wysocki" <rjw@...ysocki.net>,
	Kevin Hilman <khilman@...nel.org>,
	Magnus Damm <damm@...nsource.se>,
	Laurent Pinchart <laurent.pinchart+renesas@...asonboard.com>,
	"linux-pm@...r.kernel.org" <linux-pm@...r.kernel.org>,
	Linux-sh list <linux-sh@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 2/2] PM / Domains: Avoid infinite loops in attach/detach code

[...]

>>>
>>> @@ -2183,6 +2191,7 @@ int genpd_dev_pm_attach(struct device *dev)
>>>  {
>>>         struct of_phandle_args pd_args;
>>>         struct generic_pm_domain *pd;
>>> +       unsigned int i;
>>>         int ret;
>>>
>>>         if (!dev->of_node)
>>> @@ -2218,10 +2227,13 @@ int genpd_dev_pm_attach(struct device *dev)
>>>
>>>         dev_dbg(dev, "adding to PM domain %s\n", pd->name);
>>>
>>> -       while (1) {
>>> +       for (i = 0; i < GENPD_RETRIES; i++) {
>>>                 ret = pm_genpd_add_device(pd, dev);
>>>                 if (ret != -EAGAIN)
>>>                         break;
>>> +
>>> +               if (i > GENPD_RETRIES / 2)
>>> +                       udelay(GENPD_DELAY_US);
>>
>> In this execution path, we retry when getting -EAGAIN while believing
>> the reason to the error are only *temporary* as we are soon waiting
>> for all devices in the genpd to be system PM resumed. At least that's
>> my understanding to why we want to deal with -EAGAIN here, but I might
>> be wrong.
>>
>> In this regards, I wonder whether it could be better to re-try only a
>> few times but with a far longer interval time than a couple us. What
>> do you think?
>
> That's indeed viable. I have no idea for how long this temporary state can
> extend.

That will depend on the system PM resume time for the devices residing
in the genpd. So, I guess we need a guestimate then. How about a total
sleep time of a few seconds?

>
>> However, what if the reason to why we get -EAGAIN isn't *temporary*,
>> because we are about to enter system PM suspend state. Then the caller
>> of this function which comes via some bus' ->probe(), will hang until
>> the a system PM resume is completed. Is that really going to work? So,
>> for this case your limited re-try approach will affect this scenario
>> as well, have you considered that?
>
> There's a limit on the number of retries, so it won't hang indefinitely.

What happens with the timer functions (like msleep()) during the
system PM suspend transition?

Kind regards
Uffe
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